Are there side effects of taking 1,000 micrograms of vitamin B-12?

While the most effective daily oral dose for treating vitamin B-12 deficiency is between 647 to 1,032 micrograms, pregnant or breastfeeding women and individuals with megaloblastic anemia or Leber’s disease should consult their doctors about possible serious side effects, according to WebMD. Cobalt-sensitive individuals should not take vitamin B-12.

Patients who have recently received a coronary stent should not take vitamin B-12 in conjunction with vitamin B-6 and folate to avoid the risk of narrowing blood vessels, cautions WebMD. Vitamin B-12 is likely safe for most individuals when taken in typical supplemental doses ranging from 1 to 25 micrograms daily. Doses of up to 100 micrograms per day are sometimes used to maintain blood levels in the elderly, many of who lose the ability to absorb the vitamin from food.